TED-AJ03-298 Estimate of Mechanics property of the CO_2 Hydrate Membrane Strength

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As a promising measure to mitigate the global warming, various ideas to sequestrate CO_2 directly in the vast ocean have been proposed in the last decade. One possible scenario is to store liquid CO_2 in a dented ocean floor deeper than 3,500 m where CO_2 becomes gravitationally stable. In the conditions of higher pressures than 4.4 MPa and lower temperatures than 9.1 Celsius, however, liquid CO_2 forms clathrate hydrate, in which a CO_2 molecule is confined in a cage made of about 7 water molecules. Usually CO_2 hydrate is formed as a thin membrane at the interface between seawater and liquid CO_2. It was expected that the membrane could restrain the dissolution of stored liquid CO_2. However , it was found that CO_2 hydrate gradually dissolves into seawater. Therefore, seawater above the hydrate membrane finally becomes saturated with CO_2. Then the mechanical characteristics of CO_2 hydrate in CO_2 rich water is indispensable to evaluate the concept of CO_2 ocean storage. The strength of CO_2 hydrate membrane in CO_2 rich water was measured by Du-Nouy type surface tension meter under the condition 45-50 MPa. As a result, the membrane strength at the saturated condition shows 10-15 times stronger than the case of fresh water. In addition, the membrane shows abnormal strength and stiffness compared with that in un-saturated. Yet, the strength drastically decreases with departure from the saturated condition. The abnormality of the membrane strength just below the dissociation temperatures, which had been found in fresh water and the seawater, also appeared in this experiment. However, its peak decreases with the increase of CO_2 concentration. At first, the strength of CO_2 hydrate membrane in fresh water was discussed. The abnormal strength just below the dissociation temperatures was quantitatively explained with the idea that the diffusion of H_2O molecules is derived from Gibbs-Free-Energy. Secondly, the decrease of the abnormal strength in CO_2 rich water was quantitatively explained by the diffusion of H_2O molecules based on the hypothesis that the structure of hydrate crystal varies. At last, the stiffness of the hydrate membrane at the saturated condition was also qualitatively explained base on the diffusion of CO_2 molecules considering the distribution of CO_2,concentration in the hydrate membrane.
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TED-AJ03-298 Estimate of Mechanics property of the CO_2 Hydrate Membrane Strength

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